Assemblies of electrochemical units connected in series (often called stacks) are known. The electrochemical units thus assembled may be formed for example by accumulator elements, or by fuel cells. A fuel cell is an electrochemical device for converting chemical energy directly into electrical energy. For example, one type of fuel cell includes an anode and a cathode between which a proton exchange membrane is arranged, often called a polymer electrolyte membrane. This type of membrane only allows protons to pass between the anode and the cathode of the fuel cell. At the anode, diatomic hydrogen undergoes a reaction to produce H+ ions which will pass through the polymer electrolyte membrane. The electrons generated by this reaction join the cathode by a circuit external to the fuel cell, thus generating an electric current.
Because a single fuel cell generally only produces a low voltage (around 1 Volt), fuel cells are often series-connected to form fuel cell stacks able to generate a higher voltage comprising the sum of the voltages of each cell. One drawback of fuel cell stacks is that disconnecting them is not sufficient to stop them. Indeed, if the current supplied at output by a fuel cell is suddenly reduced to zero, the fuel cells which form the stack are no longer able to eliminate the electrochemical energy they are producing, and the voltage across the terminals of the different cells is liable to rise to the point that it accelerates degradation of the polymer membrane and the catalysts associated therewith. It is not sufficient either to interrupt the supply of fuel and oxidant to stop a fuel cell stack. In this case, the quantity of fuel and oxidant enclosed within the stack is sufficient to maintain the reaction for a considerable period of time. In the case of a fuel cell stack that uses hydrogen as fuel and oxygen as oxidant, it may even take several hours for the stack to stop.
US Patent No. 2008/0038595 discloses a method for stopping fuel cell stacks. This prior art method starts upon reception of a stop control signal. The first step of the method consists in cutting off the oxygen supply. The second step of the method consists of producing a sustained current so as to use most of the oxygen present in the stack. The third step consists in introducing air into the oxygen conduits and, finally the last step consists in cutting off the hydrogen supply. Experience has shown that, during the operation of a fuel cell stack, the various cells do not all behave in exactly the same manner. In particular, they do not all deliver the same voltage and do not release the same quantity of heat either. Thus, during the second step of the above stop method, it is advantageous to individually adjust the currents produced from each cell of the stack.
Systems are already known for shunting the current passing through certain cells in a fuel cell stack. These systems are used for stopping a fuel cell stack or for isolating a defective cell.
A known system uses a network of diodes and valves as shown in FIG. 1. Each cell includes a diode and a gas supply valve. The diode and valve are electronically controlled by a calculation unit. When the calculation unit receives a stop control signal, it short-circuits the cells via the diode of each cell. At the same time, the calculation unit cuts off the gas supply valve or valves of the cell(s) concerned in order to stop the process.
Known systems for shunting the current passing through certain cells in a fuel cell stack have some drawbacks. In particular, certain fuel cell stacks include more than a hundred series-connected cells. However, depending upon the connected load and whether or not the state of a cell is good, the voltage from an individual cell may fluctuate between 0 and around 1.2 volts. Thus, in the case of a fuel cell stack, the potential difference between certain cells and earth may amount to several tens, or even hundreds of volts. Unfortunately, the ordinary semiconductor devices normally used for controlling the current shunt cannot withstand high voltages (higher than 12 or 18 volts) between their inputs and earth.